vpp/docs/developer/corefeatures/eventviewer.rst

.. _eventviewer:

Event-logger
============

The vppinfra event logger provides very lightweight (sub-100ns)
precisely time-stamped event-logging services. See
./src/vppinfra/{elog.c, elog.h}

Serialization support makes it easy to save and ultimately to combine a
set of event logs. In a distributed system running NTP over a local LAN,
we find that event logs collected from multiple system elements can be
combined with a temporal uncertainty no worse than 50us.

A typical event definition and logging call looks like this:

.. code-block:: c

    ELOG_TYPE_DECLARE (e) =
    {
      .format = "tx-msg: stream %d local seq %d attempt %d",
      .format_args = "i4i4i4",
    };
    struct { u32 stream_id, local_sequence, retry_count; } * ed;
    ed = ELOG_DATA (m->elog_main, e);
    ed->stream_id = stream_id;
    ed->local_sequence = local_sequence;
    ed->retry_count = retry_count;

The ELOG\_DATA macro returns a pointer to 20 bytes worth of arbitrary
event data, to be formatted (offline, not at runtime) as described by
format\_args. Aside from obvious integer formats, the CLIB event logger
provides a couple of interesting additions. The "t4" format
pretty-prints enumerated values:

.. code-block:: c

    ELOG_TYPE_DECLARE (e) =
    {
      .format = "get_or_create: %s",
      .format_args = "t4",
      .n_enum_strings = 2,
      .enum_strings = { "old", "new", },
    };

The "t" format specifier indicates that the corresponding datum is an
index in the event's set of enumerated strings, as shown in the previous
event type definition.

The “T” format specifier indicates that the corresponding datum is an
index in the event log’s string heap. This allows the programmer to emit
arbitrary formatted strings. One often combines this facility with a
hash table to keep the event-log string heap from growing arbitrarily
large.

Noting the 20-octet limit per-log-entry data field, the event log
formatter supports arbitrary combinations of these data types. As in:
the ".format" field may contain one or more instances of the following:

-   i1 - 8-bit unsigned integer
-   i2 - 16-bit unsigned integer
-   i4 - 32-bit unsigned integer
-   i8 - 64-bit unsigned integer
-   f4 - float
-   f8 - double
-   s - NULL-terminated string - be careful
-   sN - N-byte character array
-   t1,2,4 - per-event enumeration ID
-   T4 - Event-log string table offset

The vpp engine event log is thread-safe, and is shared by all threads.
Take care not to serialize the computation. Although the event-logger is
about as fast as practicable, it's not appropriate for per-packet use in
hard-core data plane code. It's most appropriate for capturing rare
events - link up-down events, specific control-plane events and so
forth.

The vpp engine has several debug CLI commands for manipulating its event
log:

.. code-block:: console

    vpp# event-logger clear
    vpp# event-logger save <filename> # for security, writes into /tmp/<filename>.
                                      # <filename> must not contain '.' or '/' characters
    vpp# show event-logger [all] [<nnn>] # display the event log
                                       # by default, the last 250 entries

The event log defaults to 128K entries. The command-line argument "...
vlib { elog-events nnn } ..." configures the size of the event log.

As described above, the vpp engine event log is thread-safe and shared.
To avoid confusing non-appearance of events logged by worker threads,
make sure to code vlib\_global\_main.elog\_main - instead of
vm->elog\_main. The latter form is correct in the main thread, but
will almost certainly produce bad results in worker threads.

G2 graphical event viewer
-------------------------

The G2 graphical event viewer can display serialized vppinfra event logs
directly, or via the c2cpel tool. G2 is a fine-grained event-log viewer. It's
highly scalable, supporting O(1e7 events) and O(1e3 discrete display "tracks").
G2 displays binary data generated by the vppinfra "elog.[ch]" logger component,
and also supports the CPEL file format, as described in this section.

Building G2
~~~~~~~~~~~

This link describes :ref:`how to build G2 <building-g2>`

Setting the Display Preferences
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The file $<*HOMEDIR*>/.g2 contains display preferences, which can be overridden.
Simply un-comment one of the stanzas shown below, or experiment as desired.

.. code-block:: c

    /*
     * Property / parameter settings for G2
     *
     * Setting for a 1024x768 display:
     * event_selector_lines=20
     * drawbox_height=800
     * drawbox_width=600
     *
     * new mac w/ no monitor:
     * event_selector_lines=20
     * drawbox_height=1200
     * drawbox_width=700
     *
     * 1600x1200:
     * drawbox_width=1200
     * drawbox_height=1000
     * event_selector_lines=25
     *
     * for making screenshots on a Macbook Pro
     * drawbox_width=1200
     * drawbox_height=600
     * event_selector_lines=20
     */

Screen Taxonomy
~~~~~~~~~~~~~~~

Here is an annotated G2 viewer screenshot, corresponding to activity during BGP
prefix download. This data was captured on a Cisco IOS-XR system:

.. figure:: /_images/g21.jpg
   :scale: 75%


The viewer has two main scrollbars: the horizontal axis scrollbar shifts the main
drawing area in time; the vertical axis changes the set of visible process traces.
The zoomin / zoomout operators change the time scale.

The event selector PolyCheckMenu changes the set of displayed events.
Using these tools -- and some patience -- you can understand a given event log.

Mouse Gestures
~~~~~~~~~~~~~~~

G2 has three fairly sophisticated mouse gesture interfaces, which are worth describing
in detail. First, a left mouse click on a display event pops up a per-event detail box.

.. figure:: /_images/g22.jpg
   :scale: 75%

A left mouse click on an event detail box closes it.
To zoom to a region of the display, press and hold the left mouse button, then drag
right or left until the zoom-fence pair appears:

.. figure:: /_images/g23.jpg
   :scale: 75%

When the zoom operation completes, the display is as follows:

.. figure:: /_images/g24.jpg

A click on any of the figures will show them at full resolution, right-click will open figures in new tabs,

Time Ruler
~~~~~~~~~~

To use a time ruler, press and hold the right mouse button; drag right or left
until the ruler measures the region of interest. If the time axis scale is coarse,
event boxes can have significant width in time, so use a "reference point" in
each event box when using the time ruler.

.. figure:: /_images/g25.jpg
   :scale: 75%

Event Selection
~~~~~~~~~~~~~~~

Changing the Event Selector setup controls the set of points displayed in an
obvious way. Here, we suppress all events except "this thread is now running on the CPU":

.. figure:: /_images/g26.jpg
   :scale: 75%

Same setup, with all events displayed:

.. figure:: /_images/g27.jpg
   :scale: 75%

Note that event detail boxes previously shown, but suppressed due to deselection
of the event code will reappear when one reselects the event code. In the example
above, the "THREAD/THREADY pid:491720 tid:12" detail box appears in this fashion.

Snapshot Ring
~~~~~~~~~~~~~

Three buttons in lower left-hand corner of the g2 main window control the snapshot
ring. Snapshots are simply saved views: maneuver the viewer into an "interesting"
configuration, then press the "Snap" button to add a snapshot to the ring.

Click **Next** to restore the next available snapshot. The **Del** button deletes the current snapshot.

See the hotkey section below for access to a quick and easy method to save and
restore the snapshot ring. Eventually we may add a safe/portable/supported mechanism
to save/restore the snapshot ring from CPEL and vppinfra event log files.

Chasing Events
~~~~~~~~~~~~~~

Event chasing sorts the trace axis by occurrence of the last selected event. For
example, if one selects an event which means "thread running on the CPU" the first
N displayed traces will be the first M threads to run (N <= M; a thread may run
more than once. This feature addresses analytic problems caused by the finite size of the drawing area.

In standard (NoChaseEvent) mode, it looks like only BGP threads 5 and 9 are active:

.. figure:: /_images/g28.jpg
   :scale: 75%

After pressing the ChaseEvent button, we see a different picture:

.. figure:: /_images/g29.jpg
   :scale: 75%

Burying Boring Tracks
~~~~~~~~~~~~~~~~~~~~~

The sequence <ctrl><left-mouse-click> moves the track under the mouse to the end
of the set of tracks, effectively burying it. The sequence <shift><left-mouse-click>
moves the track under the mouse to the beginning of the set of tracks. The latter
function probably isn't precisely right--I think we may eventually provide an "undo"
stack to provide precise thread exhumation.

Summary Mode
~~~~~~~~~~~~

Summary mode declutters the screen by rendering events as short vertical line
segments instead of numbered boxes. Event detail display is unaffected. G2 starts
in summary mode, zoomed out sufficiently for all events in the trace to be displayed.
Given a large number of events, summary mode reduces initial screen-paint time to a
tolerable value. Once you've zoomed in sufficiently, type "e" - enter event mode,
to enable boxed numeric event display.

Hotkeys
~~~~~~~

G2 supports the following hotkey actions, supposedly (circa 1996) Quake-like
according to the feature's original author:

+----------------------+--------------------------------------------------------+
| Key                  | Function                                               |
+======================+========================================================+
| w                    | Zoom-in                                                |
+----------------------+--------------------------------------------------------+
| s                    | Zoom-out                                               |
+----------------------+--------------------------------------------------------+
| a                    | Scroll Left                                            |
+----------------------+--------------------------------------------------------+
| d                    | Scroll Right                                           |
+----------------------+--------------------------------------------------------+
| e                    | Toggle between event and summary-event mode            |
+----------------------+--------------------------------------------------------+
| p                    | Put (write) snapshot ring to snapshots.g2              |
+----------------------+--------------------------------------------------------+
| l                    | Load (read) snapshot ring from snapshots.g2            |
+----------------------+--------------------------------------------------------+
| <ctrl>-q             | quit                                                   |
+----------------------+--------------------------------------------------------+